The present invention relates generally to the field of digital signal processing. Specifically, the present invention provides an effective, low-delay apparatus for transcoding to provide transmission flexibility to pre-encoded bit streams by reducing characteristic bit rates according to either channel capacity or user demand.
Numerous digital video transmission techniques have been used or proposed. Although recent developments may continue to increase the capabilities of digital storage media and the bandwidth of communications channels, most conventional applications still remain constrained by the economic impracticality of storing and transmitting large amounts of electronic information. For example, conventional techniques such as NTSC (National Television Systems Committee) video generally requires large transmission rates such as about 168 Mbits per second of uncompressed video information. In response, technologists turned to digital video compression technologies in order to eliminate any processing bottleneck caused by the transmission at such large transmission rates. Even though conventional compressed digital video technologies offer comparable picture quality and require less transmission bandwidth of conventional analog video, there are still many limitations with these technologies such as the lack of standards.
Responding to a need for standardization of compressed digital video formats, the International Organization for Standardization (ISO) adopted a standard commonly called the MPEG-2 standard protocol for combining one or more "elementary streams" of coded video, audio, or other data into a single bitstream suitable for transmission. The MPEG-2 Systems standard provides a common syntax and set of semantic rules for the construction of bitstreams containing a multiplexed combination of one or more "programs." In particular, a "program" is often composed of one or more related elementary streams. An "elementary stream" is the coded representation of a single video, audio or other data stream that shares the common time base of the program of which it is a member. For example, a network television broadcast is a program having two elementary streams: a video stream and an audio stream.
As development of the MPEG-2 Systems standard progressed, a two-level packet-based multiplexing scheme emerged. At the first level, each elementary stream to be transmitted, i.e., the coded data for one video, audio or other data stream, is packetized to form a Packetized Elementary Stream (PES). Each PES packet in a given Packetized Elementary Stream includes a PES packet header followed by a variable length payload containing the coded data of that elementary stream. The Packetized Elementary Stream structure generally provides a mechanism for packaging subparts of a longer elementary stream into consecutive packets along with associated indicators and overhead information used to synchronize the presentation of that elementary stream with other, related elementary streams (e.g., elementary streams of the same program).
At the second level, one or more Packetized Elementary Streams may be further segmented or "packetized" to facilitate combining those streams into a single bitstream for transmission over some medium. Ultimately, two different second level protocols for combining one or more Packetized Elementary Streams into a single bitstream emerged: 1) the Program Stream (PS) protocol and 2) the Transport Stream protocol. Both stream protocols are packet-based and fall into the category of transport layer entities, as defined by the ISO Open System Interconnection (OSI) reference model. Program Streams utilize variable-length packets and are intended for error-free environments in which software parsing is desired. Program Stream packets are generally relatively large (e.g., 1K to 2K bytes). Transport Streams utilize fixed length packets and are intended for transmission in noisy or error prone environments. Each Transport Stream packet comprises a header portion and a payload portion. Transport Stream packets have a relatively short length of about 188 bytes and include features for enhanced error resiliency and packet loss detection. Each group of Transport Stream packets that contain the same elementary stream data are assigned the same unique Packet ID (PID). For example, the elementary stream containing the coded video data for a network television program may be assigned a PID of "10"; the elementary stream containing the associated audio data for that program may be assigned a PID of "23," and so on.
The MPEG standard adopts a model of compression and decompression in which interframe redundancy is first removed from the color motion picture frames. To achieve interframe redundancy removal, each frames is designated either "intra" "predicted" or "bidirectional" for coding purposes. Intra frames are least frequently provided, the predicted frames are provided more frequently than the intra frames, and remaining frames are bidirectional frames. Values for every pixel in an intra frame (I) are independently provided. In a prediction frame (P), only the incremental changes in pixel values from the last I-picture or P-picture are coded. In a bidirectional frame (B), the pixel values are coded with respect to both an earlier frame and a later frame. The MPEG standard does not require frames to be stored in strict time sequence, such that the intraframe from which a predicted frame is coded can be provided in the picture sequence either earlier or later in time as the predicted frame.
Limitations arise, however, when a compressed video stream having a characteristic data rate must be transmitted through a channel of constrained bandwidth, or transmitted by hardware operating at a lower data rate. That is, conventional transmission technologies generally cannot accurately edit the compressed video steam in real time to provide a "clear" video image at a user location. Conventional transmission technologies for video applications are also extremely costly and cannot be implemented easily.
What is needed is a cost effective, apparatus for transcoding to provide transmission flexibility to pre-encoded bit streams by reducing the characteristic bit rates according to either channel capacity or user demand.